1. Field of the Invention
This invention relates to digital encoding arrangements of the type which convert the position location of a movable device into a digital electric signal suitable for processing by digital computer. More particularly, this invention relates to the digital encoding of movable devices such as the keyboard keys and pedals of a musical organ or synthesizer.
2. Description of the Prior Art
A first method known in the prior art for generating digital data responsive to the position of a movable device, such as a key in the keyboard of a musical instrument, provides mechanical coupling between the key and a respectively associated two-state switch. Depression of the key causes the switch to change states, thereby affording detection of only two key positions--up and down. Circuitry may be provided to sequentially scan the switches to determine each one's binary state and transmit the information to a computer. This arrangement, however, suffers from the substantial shortcoming of invariable threshold. Transient and intermediate key positions cannot be encoded, and the arrangement is unsuitable for the generation of touch-responsive dynamic data.
The prior art provides schemes which generate signals responsive to the transient location of a movable member. A first such scheme relates to the mechanical coupling of a movable device to an optical fiber. One end of the fiber is affixed to a stationary support and the other end follows a predetermined path as the movable device is moved. A source of light is located near the stationary end of the fiber so as to permit the light to enter the fiber at the stationary end and exit at the moving end. Plural photoelectric cells are located near the path of the moving end of the optical fiber and receive the light emitted from the fiber as the movable device moves. The resolution of such an arrangement is limited by the number of photoelectric cells which can be situated near the path of the moving end of the fiber. Typically, ten or more photoelectric cells are required for each movable device. The cost and interconnection complexity of this arrangement when used in a system containing over 250 movable devices is obviously prohibitive.
Magnetostrictive delay lines have been used in the prior art to generate digital data responsive to the location of a movable device. Briefly, a movable device with an associated pickup coil travels along the length of the delay line. An electrical signal initiates a propagating strain wave along the delay line and starts a clocked counter. When the strain wave passes the point on the magnetostrictive delay line where the pickup coil is located, a signal induced in the coil stops the counter. The digital number contained in the counter at the time it is stopped is representative of the location of the pickup coil along the delay line. This magnetostrictive approach to the digital encoding of analog positions requires many such delay lines and associated hardware in a system for encoding many movable devices. Furthermore, since the rate of strain wave propagation along the delay line is a function of its geometry and age, precise correlation between the various delay lines is virtually impossible.
It is, therefore, an object of this invention to generate digital signals responsive to the instantaneous position location of each of a multiplicity of separate movable devices, irrespective of whether each device is at rest or in transition.
It is a further object of this invention to provide an economical system for generating digital signals responsive to the position location of each of a multiplicity of separate movable devices relative to a reference common to all such devices.